ffmpeg: make -lavfi an alias for -filter_complex.
fate: add a test for -filter_complex / -lavfi without input.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  fate: add a test for -filter_complex / -lavfi without input.
  ffmpeg: make -lavfi an alias for -filter_complex.

Merged-by: Michael Niedermayer <michaelni@gmx.at>